Urea is the major end product of nitrogen metabolism in mammals. In the kidney, urea transport plays a central role in the urinary concentration mechanism and in water conservation. Transport of urea, regulated by vasopressin, is via a facilitated transporter located in the inner medulla collecting duct ("IMCD") of the kidney. Urea transporters situated in the plasma membranes of other epithelia including colon, liver, and lung, are crucial for urea and nitrogen metabolism.
It has been very difficult to obtain reliable information about the identity of urea transporters. There are presently no known effective urea transport inhibitors. Furthermore, urea transporters are very hydrophobic, making them hard to isolate and purify.